A charge pump is a circuit that can generate an output voltage that is higher than the voltage supplied to the charge pump. One of the applications of charge pumps is to develop voltage for erasing and programming some kinds of nonvolatile semiconductor memory devices, such as electrical erasable programmable read only memory (EEPROM) and flash memory. One way to operate these memory devices is to program through hot electron injection and erase through Fowler-Nordheim tunneling. The programming and erasing of such a memory cell require current to pass through the dielectric surrounding a floating gate electrode. As a result, a high voltage is generally needed. Some prior art nonvolatile semiconductor memory devices require the application of an external high voltage (e.g, 12 volts) in addition to a regular 5 volts supply voltage. This arrangement is undesirable because it is complicated and wastes real estate on circuit boards. Recently, many nonvolatile semiconductor memory device manufacturers place charge pumps on chip so as to develop the required high voltage for erasing and programming. Many customers welcome this development, and the sale of nonvolatile semiconductor devices increases.
As the number of memory cells in a nonvolatile semiconductor memory device increases, the current required to erase and program these cells also increases. As a result, there is a need for the charge pump to be efficient, e.g., generating more current and at a faster rate.